As per IRC discussion, enabling dub linting appears to break things at a certain point, the build buttons disappear and code completion stops working. Looking at the output in Developer Tools, the problem seems to start here:
[Extension Host] Initialized code-d
[Extension Host] dub is ready
[Extension Host] DScanner is ready
[Extension Host] Dfmt is ready
[Extension Host] DlangUI is ready
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:11:59.064:server.d:main:133 Starting up...
Server: 2016-05-17T19:11:59.064:server.d:loadConfiguredImportDirs:406 Loading configuration from /home/gnunn/.config/dcd/dcd.conf
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:11:59.065:server.d:main:167 Listening at /tmp/workspace-d-sock4Q6
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:11:59.107:first.d:visit:387 Could not resolve location of module 'core/sys/windows/winsock'
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:11:59.497:first.d:visit:387 Could not resolve location of module 'ldc/intrinsics'
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:11:59.652:first.d:visit:387 Could not resolve location of module 'gcc/builtins'
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:01.618:first.d:visit:387 Could not resolve location of module 'tango/stdc/time'
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:01.681:first.d:visit:387 Could not resolve location of module 'tango/stdc/stdio'
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:01.722:first.d:visit:387 Could not resolve location of module 'core/sys/windows/winsock'
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:01.727:first.d:visit:387 Could not resolve location of module 'core/sys/windows/winsock'
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:01.870:server.d:main:184 Import directories:
Server: /home/gnunn/Development/dlang/terminix/source/
Server: /home/gnunn/.dub/packages/gtk-d-3.3.1/srcgstreamer/
Server: /home/gnunn/.dub/packages/gtk-d-3.3.1/src/
Server: /home/gnunn/.dub/packages/gtk-d-3.3.1/srcgl/
Server: /home/gnunn/.dub/packages/gtk-d-3.3.1/srcsv/
Server: /home/gnunn/.dub/packages/gtk-d-3.3.1/srcvte/
Server: /usr/include/dlang/dmd
Server: /usr/include/dlang/dmd
Server: 2016-05-17T19:12:01.870:server.d:main:190 164973 symbols cached.
Server: 2016-05-17T19:12:01.870:server.d:main:191 Startup completed in 2806.15 milliseconds.
[Extension Host] DCD is ready
[Extension Host] ["/home/gnunn/Development/dlang/terminix/source/", "/home/gnunn/.dub/packages/gtk-d-3.3.1/srcgstreamer/", "/home/gnunn/.dub/packages/gtk-d-3.3.1/src/", "/home/gnunn/.dub/packages/gtk-d-3.3.1/srcgl/", "/home/gnunn/.dub/packages/gtk-d-3.3.1/srcsv/", "/home/gnunn/.dub/packages/gtk-d-3.3.1/srcvte/"]
[Extension Host] provideHover
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:22.043:server.d:main:290 Getting doc comment
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:22.044:server.d:main:321 Request processed in 0.876729 milliseconds
[Extension Host] resolve null
[Extension Host] provideHover
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:28.855:server.d:main:290 Getting doc comment
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:28.856:server.d:main:321 Request processed in 0.569216 milliseconds
[Extension Host] resolve null
[Extension Host] provideHover
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:30.114:server.d:main:290 Getting doc comment
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:30.114:autocomplete.d:getSymbolsByTokenChain:746 Could not find declaration of "test3" from position 792
Server: 2016-05-17T19:12:30.114:autocomplete.d:getDoc:71 Could not find symbol
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:30.114:server.d:main:321 Request processed in 0.80036 milliseconds
[Extension Host] resolve null
[Extension Host] provideHover
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:30.592:server.d:main:290 Getting doc comment
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:30.593:autocomplete.d:getSymbolsByTokenChain:746 Could not find declaration of "test3" from position 792
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:30.593:autocomplete.d:getDoc:71 Could not find symbol
Server: 2016-05-17T19:12:30.593:server.d:main:321 Request processed in 0.819946 milliseconds
[Extension Host] resolve null
[Extension Host] provideCompletionItems
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:37.856:server.d:main:283 Getting completions
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:37.917:server.d:main:321 Request processed in 60.806 milliseconds
[Extension Host] resolve
[Extension Host] [Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object…]
[Extension Host] provideCompletionItems
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:39.082:server.d:main:283 Getting completions
[Extension Host] WorkspaceD Debug: Server: 2016-05-17T19:12:39.090:server.d:main:321 Request processed in 8.37959 milliseconds
[Extension Host] resolve
[Extension Host] [Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object…]
[Extension Host] lint
[Extension Host] dubBuild
[Extension Host] [Object, Object]
[Extension Host] [Object, Object]
[Extension Host] [Object, Object]
[Extension Host] Resolve
[Extension Host] [Array[2]]
[Extension Host] Resolve
[Extension Host] [Array[2]]
[Extension Host] lint
[Extension Host] dubBuild
[Extension Host] Resolve
[Extension Host] [Array[2]]
[Extension Host] WorkspaceD Debug: .dub/build/default-debug-linux.posix-x86_64-dmd_2071-86ACB15FB45823A99BC30ACB1A6FB141/terminix.o:(.rodata+0x5c68): undefined reference to `_D4glib9FileUtils12__ModuleInfoZ'
.dub/build/default-debug-linux.posix-x86_64-dmd_2071-86ACB15FB45823A99BC30ACB1A6FB141/terminix.o:(.rodata+0x5c70): undefined reference to `_D4glib4Util12__ModuleInfoZ'
.dub/build/default-debug-linux.posix-x86_64-dmd_2071-86ACB15FB45823A99BC30ACB1A6FB141/terminix.o:(.rodata+0x5c78): undefined reference to `_D3gtk4Main12__ModuleInfoZ'
.dub/build/default-debug-linux.posix-x86_64-dmd_2071-86ACB15FB45823A99BC30ACB1A6FB141/terminix.o:(.rodata+0x5c80): undefined reference to `_D3gtk7Version12__ModuleInfoZ'
.dub/build/default-debug-linux.posix-x86_64-dmd_2071-86ACB15FB45823A99BC30ACB1A6FB141/terminix.o:(.rodata+0x5c88): undefined reference to `_D3gtk13MessageDialog12__ModuleInfoZ'
.dub/build/default-debug-linux.posix-x86_64-dmd_2071-86ACB15FB45823A99BC30ACB1A6FB141/terminix.o:(.rodata+0x5cf0): undefined reference to `_D3gio14DBusConnection12__ModuleInfoZ'
.dub/build/default-debug-linux.posix-x86_64-dmd_2071-86ACB15FB45823A99BC30ACB1A6FB141/terminix.o:(.rodata+0x5da8): undefined reference to `_D3gio11ActionMapIF12__ModuleInfoZ'
.dub/build/default-debug-linux.posix-x86_64-dmd_2071-86ACB15FB45823A99BC30ACB1A6FB141/terminix.o:(.rodata+0x5db0): undefined reference to `_D3gio12SimpleAction12__ModuleInfoZ'
...
Those error messages continue on for every GtkD class and generate about 13000 lines.